Each class rolls a certain dice upon gaining a level to determine how much HP they gain.
Sorcs/Wizards-1d4 (1-4 HP)
Bards/Rogues-1d6
Druids/Clerics-1d8
Fighters/Rangers-1d10
Barbarians-1d12
After the dice is rolled, the characters constitution modifer is added, to determine the final amount of HP gained.
For example, say a Barbarian has a constitution of 18, which grants a +4 modifier. He rolls a d12, and gets an 9. 9 From the roll+4 from his con modifer= 13 total HP gained.
Hope that helps